Parkinson's disease is widely considered to be the result of degradation of the pre-synaptic dopaminergic neurons in the brain, with a subsequent decrease in the amount of the neurotransmitter dopamine, that is being released. Inadequate dopamine release, therefore, leads to the onset of voluntary muscle control disturbances symptomatic of Parkinson's disease.
Various procedures for treating Parkinson's disease have been established and are currently in widespread use, for example, the administration of L-Dopa together with a decarboxylase inhibitor, such as L-carbidopa or benzerazide. The decarboxylase inhibitor protects the L-Dopa molecule from peripheral decarboxylation and thus ensures L-Dopa uptake by the remaining dopaminergic neurons in the striatum of the brain. Here the L-Dopa is converted into dopamine resulting in increased levels of dopamine in these neurons. In response to physiological impulses these neurons are therefore capable of releasing larger amounts of dopamine, the quantity of which approximates the normal required levels. This treatment therefore alleviates the symptoms of the disease and contributes to the well-being of the patients.
However, this L-Dopa treatment has its drawbacks, the main one being that its effectiveness is optimal only in the first few years following the onset of treatment. After this initial period the clinical response is diminished and is accompanied by adverse side effects which include dyskinesia, fluctuation in efficacy throughout the day ("on-off effect") and psychiatric symptoms such as confusional states, paranoia and hallucinations. This fall-off in the effect of L-Dopa treatment is attributed to a number of factors, including the natural progression of the disease, alteration in dopamine receptors as a consequence of increased dopamine production or increased levels of dopamine metabolites, and pharmacokinetic problems of L-Dopa absorption (reviewed by Youdim et al., Progress in Medicinal Chemistry, Vol. 21, Chapter 4, pp. 138-167 (1984), Eds. Ellis and West, Elsevier, Amsterdam).
In order to overcome the drawbacks of the L-Dopa treatment, various treatments have been devised in which L-Dopa is combined with MAO inhibitors, with the aim of reducing the metabolic breakdown of the newly formed dopamine (see for example, U.S. Pat. No. 4,826,875).
MAO exists in two forms known as MAO-A and MAO-B which have selectivity for different substrates and inhibitors. For example, MAO-B metabolizes more efficiently substrates such as 2-phenylethylamine and is selectively and irreversibly inhibited by (-)-deprenyl (as described below).
It should be noted, however, that combining L-Dopa with an inhibitor of both MAO-A and MAO-B is undesirable leading to adverse side effects related to an increased level of catecholamines throughout the neuraxis. Furthermore, complete inhibition of MAO is also undesirable as it potentiates the action of sympathomimetic amines such as tyramine leading to the so-called "cheese effect" (reviewed by Youdim et al., Handbook of Experimental Pharmacology, Vol. 90, Chap. 3 (1988) Eds, Trendelenburg and Weiner, Springer-Verlag). As MAO-B was shown to be the predominant form of MAO in the brain, selective inhibitors for this form were thus considered to be a possible way for achieving a decrease in dopamine breakdown on the one hand, together with a minimization of the systemic effects of total MAO inhibition, on the other.
One of these selective MAO-B inhibitors, (-)-deprenyl, has been extensively studied and has been used as an MAO-B inhibitor to augment L-Dopa treatment. This treatment with (-)-deprenyl is generally favorable, not causing the "cheese effect" at doses causing nearly complete inhibition of MAO-B (Elsworth et al., Physchopharmacology, 57, 33 (1978). Furthermore, addition of (-)-deprenyl to a combination of L-Dopa and decarboxylase inhibitor to Parkinson's patients leads to improvements in akinesia and overall functional capacity as well as the elimination of "on-off" type fluctuations (reviewed by Birkmayer & Riederer in "Parkinson's Disease" pp. 138-149, Springer-Verlag (1983)).
Thus, (-)-deprenyl enhances and prolongs the effect of L-Dopa and permits-a lowering of the dosage of L-Dopa whereby the adverse effects of L-Dopa treatment are limited.
However, (-)-deprenyl is not without its own adverse sides effects which include activation of pre-existing gastric ulcers and occasional hypertensive episodes. Furthermore, (-)-deprenyl is an amphetamine derivative and is metabolized to yield amphetamine and methamphetamines which may lead to undesirable side effects associated with these substances, e.g. increased heart rate (Simpson, Biochemical Pharmacology, 27, 1591 (1978); Finberg et al., in "Monoamine Oxidase Inhibitors--The State of the Art", pp. 31-43, Eds. Youdim and Paykel, (1981) Wiley).
Other compounds that are selective irreversible inhibitors of MAO-B but which are free of the undesirable effects associated with (-)-deprenyl have been described. One such compound, namely N-propargyl-l-aminoindan. HCl (racemic-PAI.HCl) was described in GB 1,003,686, GB 1,037,014 and U.S. Pat. No. 3,513,244. It is a potent, selective, irreversible inhibitor of MAO-B, is not metabolized to amphetamines and does not give rise to unwanted sympathomimetic effects.
In comparative animal tests racemic PAI was shown to have considerable advantages over (-)-deprenyl, for example, racemic PAI produced no significant tachycardia, did not increase blood pressure (effects produced by doses of 5 mg/kg of (-)-deprenyl), and did not lead to contraction of nictitating membrane nor to an increase in heart rate at doses up to 5 mg/kg (effects caused by (-)-deprenyl at doses over 0. 5 mg/kg). Furthermore, racemic PAI.HCl does not potentiate the cardiovascular effects of tyramine (Finberg et al. in "Enzymes and Neurotransmitters in Mental Disease", pp. 205-219, (1980), Eds. Usdin et al., Pub. John Wiley and Sons, N.Y.; Finberg et al. (1981) in "Monoamine Oxidase Inhibitors--The State of the Art", ibid; Finberg and Youdim, British Journal Pharmacol. 85 451, (1985).
One object of this invention is to separate the racemic PAI compounds and to produce an enantiomer with MAO-B inhibition activity.
Since deprenyl has a similar structure to PAI and it is known that the (-)-enantiomer of deprenyl, i.e. (-)-deprenyl, is considerably more pharmaceutically active than the (+)-enantiomer, it was expected, by those skilled in the art, that only the (-) enantiomer of PAI would be the active MAO-B inhibitor.
However, contrary to such expectations, upon resolution of the enantiomers, it was found, in accordance with the present invention that the (+)-PAI enantiomer was in fact the active MAO-B inhibitor while the (-)enantiomer showed extremely low MAO-B inhibitory activity. Furthermore, the (+)PAI enantiomer surprisingly also had a higher degree of selectivity for MAO-B inhibition than the corresponding racemic form and may thus have less undesirable side effects in the treatment of the indicated disease. These findings are based on both in vitro and in vivo experiments as presented hereinafter in greater detail.
It was subsequently shown that (+)-PAI has the R absolute configuration. This was also surprising based on the expected structural analogy with deprenyl and the amphetamines.
The high degree of stereoselectivity of pharmacological activity between R(+)-PAI and the S(-) enantiomer is also remarkable. The compounds R(+)-PAI is nearly four orders of magnitude more active than the S(-) enantiomer in MAO-B inhibition. This ratio is significantly higher than that observed between the two deprenyl enantiomers (Knoll and Magyar, Adv. Biochem. Physchopharmacol., 5, 393 (1972); Magyar, et al., Acta Physiol. Acad. Sci. Hung., 32, 377 (1967). Furthermore, in some physiological tests, (+) deprenyl was reported to have equal or even higher activity than the (-) enantiomer (Tekes, et al., Pol. J. Pharmacol. Pharm. 40, 653 (1988).
N-methyl-N-propargylaminoindan (MPAI) is a more potent inhibitor of MAO activity, but with lower selectivity for MAO-B over A (Tipton, et al., Biochem. Pharmacol., 31, 1250 (1982)). Surprisingly, in this case we have found only small degree of difference in the relative activities of the two resolved enantiomers thus further emphasising the remarkableness of the case of R(+)-PAI. (See Table 1A).
Another object of the present invention is to provide for the first time use of the pharmaceutically active PAI-enantiomer alone (without L-Dopa) for treatment of Parkinson's disease, dementia and depression (see review by Youdim et al. in Handbook of Experimental Pharmacology, Vol. 90/I, (1988), chap.3, Eds. Trendelenberg and Wiener).
It is yet another object of the invention to provide for the use of the pharmaceutically active PAI-enantiomer for pre-treatment alone or together with synergistic agents, of Parkinson's disease in order to delay the L-Dopa treatment and its associated adverse side effects. This approach has been studied with respect to (-)-deprenyl which was shown to be effective when administered alone to early Parkinsonism patients, and may also have a synergistic effect in these patients when administered together with .alpha.-tocopherol (a vitamin E derivative), (The Parkinson's Study Group, New England J. Med., 321(20), 1364-1371, (1989)).
In addition to its usefulness in treating Parkinson's disease, (-)-deprenyl has also been shown to be useful in the treatment of patients with dementia of the Alzheimer type (DAT) (Tariot et al., Psychopharmacology, 91, 489-495, 1987), and in the treatment of depression (Mendelewicz and Youdim, Brit. J. Psychiat. 142, 508-511, 1983). Thus, the R(+)-PAI compound of this invention has been shown to possess activity in restoration of memory, thus having potential for treatment of memory disorders, dementia and especially useful in Alzheimer's disease and for the treatment of the hyperactive syndrome in children.